The present invention relates to a semiconductor detector and more exactly to an integrated xcex94E-E detector.
In many branches of science there is a need to measure energetic atoms with subrelativistic energies. Examples include, mineral prospecting with accelerator mass spectrometry, spacecraft measurements of the solar wind as well as interplanetary and galactic radiation environments, nuclear microprobe analysis of light trace distributions in biomedical matter, recoil spectrometry characterization of opto- and microelectronic structures as well as fundamental experimental physics. Depending on the particular circumstances, a number of different techniques are available, such as electrostatic- and magnetic-sector spectrometers, Time of Flight techniques, scintillation detectors as well as semiconductors and gas-ionization detectors. Often the different techniques are combined, e.g., a magnetic spectrometer analyzer may be combined with a gas ionization detector to facilitate separate measurement of the momentum, atomic number and energy.
For measurement of the atomic number as well as the energy of energetic atoms and ions in the 5 MeV per nucelon to 0.1 GeV per nucleon energy region, a so-called xcex94E-E detector telescope is often used. These consist of a pair of detectors arranged so that the particle traverses the first detector and is subsequently stopped in the second (thicker) detector. The atomic number can be determined because the energy deposition within the thickness spanned by the xcex94E detector depends on the electrical charge on the nucleus of the energetic atom (atom number). The total energy is the sum of the energy deposited in the two detectors. Exactly how the deposited energy is converted into an electrical signal depends on the type of detector. In the energy region of interest almost all the energy is deposited in exciting electrons of the detector material. In the case of a gas-ionization detector the number of ion-electron pairs can be registered. In the case of solid detector materials (scintillator and semiconductors) the electronic energy deposition leads to promotion of electrons across the band gap. This leads to photon emission in the case of the scintillator or a pulse of electric current flowing across a junction, in the case of a semiconductor detector. The junction in the semiconductor detector may be a Schottky barrier or an ion-implanted or diffused junction.
Semiconductor detectors have the advantage over electrostatic and magnetic sector instruments that they are insensitive to the charge state of the incident ion or atom. However xcex94E-E detector telescopes have a low energy threshold that corresponds to the energy where the range of the ion is just sufficient to penetrate right through the xcex94E detector. To reduce the low-energy threshold as low as possible it is necessary to fabricate the xcex94E detector as thin as possible.
The xcex94E-E detector telescopes with the lowest threshold that are currently suitable for mass-production are based on a p-i-n structure formed in a self-supporting 10xc3x9710 mm area 10 xcexcm Si membrane that is formed by etching (see Whitlow et al., Proc. 9th Australian Conf. on Nuclear Techniques of Analysis, Newcastle, Australia, Nov. 1995). The p-i-n structure can be formed by ion-implantation or diffusion (see J. Kremmer, Nucl. Instr. and Methods, Vol. 169, pp 499, 1980). These have a low energy threshold of about 1A MeV and can be stacked together to form arrays subtending large solid angles with little dead area because of the rectangular shape. Thinner detectors can be produced, however, the difficulty of controlling the etching, and the use of Au/Si Schottky junctions implies the resolution is dominated by the uniformity in the active thickness of the detector. The extreme fragility of the self-supporting Si membranes places a technical restriction on the minimum thickness and thereby the low energy threshold that can be achieved. This precludes the use of self-supporting membrane detectors in space crafts because of the G-forces and vibration during the launch phase. Kremmer and Lutz have proposed an ion-implanted n-p+-n structure which is depleted from two sides (see Kremmer and Lutz, Nuclear Instrum. and Methods, A235, 365, 1978). In a further variant they propose using lateral drift fields to realize the separate collection of the xcex94E and E signals. No data is presented from measurement with this type of detector. Possibly because the typical xc2x1150% variations in the background doping will lead to significant variations in the xcex94E layer thickness that will result in poor resolution.
Integrated detector telescopes, for instance according to FIG. 1, have been reported by a Japanese group about a decade or so ago (e.g. see Husimi et al., Nuclear Instr. and Methods, Vol 196, pp 131, 1982 and Y. Kim et al, Nuclear Instr. and Methods, Vol. 226, pp 125, 1984). This is based on epitaxial growth of a p-n-p structure. This integrated detector actually worked, however, the concept suffers from a number of drawbacks. The fact that epitaxy was used implies that it is not possible to obtain a well defined buried layer that forms the buried contact to the E and xcex94E detector elements. This limits the minimum thickness of the thin detector and the thickness uniformity, which will be critically dependant on the gas dynamics during epitaxy. A potentially more serious problem is cross-coupling between the xcex94E-E detector pair because the intense ionization along the ion track can form a plasma with a carrier concentration exceeding the doping concentration in the buried contact. Under these conditions the plasma can appreciably modify the local electric field round the plasma column and acts as a path for charge carriers between the E and xcex94E detectors.
A U.S. Pat. No. 5,387,555 describes a method to create buried silicide layers with wafer bonding. However the method described contains also at least one insulating buried layer in contact with the buried silicide layer. Moreover the method according to this patent lacks a cooperating dopant for diffusion from the buried silicide layer.
Consequently it is found that there is a strong demand for an integrated semiconductor xcex94E-E detector generally offering a good mechanical stability and presenting a xcex94E-portion which may even withstand strong momentary mechanical forces, e.g. an acceleration, as well as also presenting a high resolution which will imply a very thin structure necessary for the xcex94E portion of the detector. These two main demands will then be more or less contradictory to each other.
To avoid the problems discussed above the present invention suggests a structure consisting of a number of p-i-n diodes separated by metallic layers. The metallic layer between the devices ensures that there is no coupling between the different detector elements due to charge carrier funneling and minimizes image charge effects.
According to a main object of the present invention a detector telescope is formed with a very thin xcex94E detector portion fabricated from a first silicon wafer which is bonded/silicidized to a second silicon wafer which forms the E detector portion of the detector telescope and thereby producing a well supported very thin xcex94E detector for high resolution in the detector telescope, the very thin xcex94E detector portion and the E detector portion between each other further present a buried metallic layer acting as a contact common to the two detectors, whereby the metallic layer is very thin and presents a low resistivity.
Other objects and advantages of different embodiments of the invention will be further defined in the dependent claims.